In the macro world (measured in fractions of an inch) spring loaded contacts are widely used because of their ability to absorb stresses associated with pressing two surfaces together. This is of particular importance if either, or both, surface is fragile and subject to damage if the contact force were to be transmitted to it. Contacting systems of this type are commonly formed by seating a ball in a cylindrical cavity, behind an opening that is slightly smaller than the diameter of the bail, and mounting a coil spring between the ball and the other end of the cylinder.
Similar stresses are associated with contacting systems that are used in the micro world (measured in microns and fractions thereof) but, to the best of our knowledge, spring loaded contacts similar to those described above have not yet been developed. A particular example of a micro system in which a fragile set of surfaces need to make and maintain good contact with a second set of surfaces, is a packaged integrated circuit where contact pads that are part of a chip need to make contact with solder balls located on the outside of the package. These solder balls will be used later to attach the package to the next packaging level (for example a printed circuit board) and it is important that the associated thermal and mechanical stresses not be transmitted down to the chip level.
The reason that spring loaded contacts have thus far not been used in the micro world is due, we believe, to the difficulty of fabricating a truly three dimensional object such as a coil spring (as opposed to a multi-layer structure with vertical connections between layers).
The closest prior art to (though still substantially different from) the present invention that we have found are a pair of patents by Little (U.S. Pat. No. 5,663,596--structure) and (U.S. Pat. No. 5,665,648--method). In Little's invention, chip contacting pads are located on the vertical edges of the integrated circuit chip. The latter is attached to a substrate through spring contacts that have a base portion joined to a line on the substrate and a spring portion that presses against the edge of the chip. To fabricate the spring, a release layer is first laid down. Then a metal film is deposited under conditions known to induce high tensile stress so that, when the release layer is etched away, the freed metal curls up to a sufficient degree to allow the chip to be pushed against its underside, thereby forming a spring contact.
While Little's structure is not a coil, coil-like structures have been described in the prior art for use as micro inductors. These structures are, however, not truly three dimensional and comprise multiple layers of open rings that are then connected in series by means of vertical vias. An example of this is described by Lee (U.S. Pat. No. 5,831,331). Such structures, while working as inductors, could not be used to provide stress relief as each ring is separated from its neighbors by a rigid layer. Even if the latter was made to be flexible, absorption of stress would be limited to the vias which have very little flexibility so that any, except very small, stresses would be transmitted between contacting surfaces.